Magneto-quantum oscillation experiments in high temperature superconductors show a strong thermally-induced suppression of the oscillation amplitude approaching critical dopings---in support of a quantum critical origin of their phase diagrams. We su
ggest that, in addition to a thermodynamic mass enhancement, these experiments may directly indicate the increasing role of quantum fluctuations that suppress the oscillation amplitude through inelastic scattering. We show that the traditional theoretical approaches beyond Lifshitz-Kosevich to calculate the oscillation amplitude in correlated metals result in a contradiction with the third law of thermodynamics and suggest a way to rectify this problem.
Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi_2Se_3 in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9times10^16cm^-3, the lowest Landau level of the
bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the u =1 Landau level attained by a field of 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.
The three-dimensional Fermi surface morphology of superconducting BaFe_2(As_0.37}P_0.63)_2 with T_c=9K, is determined using the de Haas-van Alphen effect (dHvA). The inner electron pocket has a similar area and k_z interplane warping to the observed
hole pocket, revealing that the Fermi surfaces are geometrically well nested in the (pi,pi) direction. These results are in stark contrast to the Fermiology of the non-superconducting phosphides (x=1), and therefore suggests an important role for nesting in pnictide superconductivity.
We measure magnetic quantum oscillations in the underdoped cuprates YBa$_2$Cu$_3$O$_{6+x}$ with $x=0.61$, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprate
s has a maximum at $papprox 0.11-0.12$. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T=0 limit of the metal-insulator crossover (low-$p$ side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high $p$ side).
We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in the low energy magneto-optical absorption spectrum opens at low temperature. This previously unreported collective phenomenon arises at temperatures where
the energy of the dominant exchange interaction exceeds the thermal energy. Simulations of the observed shifts in electron paramagnetic resonance spectral weight, which include spin anisotropy, reproduce this behavior yielding the magnitude of the spin anisotropy in these compounds.
We report the Hall resistivity, $rho_{xy}$ of polycrystalline SmFeAsO$_{1-x}$F$_{x}$ for four different fluorine concentrations from the onset of superconductivity through the collapse of the structural phase transition. For the two more highly-doped
samples, $rho_{xy}$ is linear in magnetic field up to 50 T with only weak temperature dependence, reminiscent of a simple Fermi liquid. For the lightly-doped samples with $x<0.15$, we find a low temperature regime characterized $rho_{xy}(H)$ being both non-linear in magnetic field and strongly temperature dependent even though the Hall angle is small. The onset temperature for this non-linear regime is in the vicinity of the structural phase (SPT)/spin density wave (SDW) transitions. The temperature dependence of the Hall resistivity is consistent with a thermal activation of carriers across an energy gap. The evolution of the energy gap with doping is reported.
The so-called stripe phase of the manganites is an important example of the complex behaviour of metal oxides, and has long been interpreted as the localisation of charge at atomic sites. Here, we demonstrate via resistance measurements on La_{0.50}C
a_{0.50}MnO_3 that this state is in fact a prototypical charge density wave (CDW) which undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. Our discovery that the manganite superstructure is a CDW shows that unusual transport and structural properties do not require exotic physics, but can emerge when a well-understood phase (the CDW) coexists with disorder.
We point out that a recent model for the heat capacity of alpha-U that invokes CDW collective modes is unphysical. We show instead that the features in the heat capacity of both single-crystal and polycrystalline alpha-U can be accounted for by a num
ber of Peierls transitions that are subject to increased disorder in the polycrystalline sample.
We describe instrumentation designed to perform millimeter-wave conductivity measurements in pulsed high magnetic fields at low temperatures. The main component of this system is an entirely non-metallic microwave resonator. The resonator utilizes pe
riodic dielectric arrays (photonic bandgap structures) to confine the radiation, such that the resonant modes have a high Q-factor, and the system possesses sufficient sensitivity to measure small samples within the duration of a magnet pulse. As well as measuring the sample conductivity to probe orbital physics in metallic systems, this technique can detect the sample permittivity and permeability allowing measurement of spin physics in insulating systems. We demonstrate the system performance in pulsed magnetic fields with both electron paramagnetic resonance experiments and conductivity measurements of correlated electron systems.
Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and may allow it to recover its previous shape after plastic deformation. This is k
nown as the shape-memory effect (SME). We have succeeded in identifying the primary electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. This strongly suggests that electronic band structure is an important consideration in the design of future SME alloys.
